Mass is an intrinsic property of matter.
From Newton's second law,
, we have that the amount of
force required to accelerate an object, by a given amount, is
proportional to its mass. Thus, the mass of an object quantifies its
inertia--its resistance to a change in velocity.

We can measure the mass of an object by measuring the
gravitational force between it and another known mass,
as described in the next section. This is a special case of measuring
its acceleration in response to a known force. Whatever the force
,
the mass
is given by
divided by the resulting acceleration
, again by Newton's second law
.

The usual mathematical model for an ideal mass is a dimensionless
point at some location in space. While no real objects are
dimensionless, they can often be treated mathematically as
dimensionless points located at their center of mass, or
centroid (§B.4.1).

The physical state of a mass
at time
consists of its
position
and velocity
in 3D space.
The amount of mass itself,
, is regarded as a fixed parameter that
does not change. In other words, the state
of a
physical system typically changes over time, while any
parameters of the system, such as mass
, remain fixed over
time (unless otherwise specified).